Propellant grain



1965 A. 1.. HOLZMAN ETAL 3,

PROPELLANT GRAIN 2 Sheets-Sheet 1 Filed May 6, 1965 INVENTOR.

1965 A. HOLZMAN ETAL 3,

PROPELLANT GRAIN 2 Sheets-Sheet 2 Filed May 6, 1963 INVE'NTOR.

United States Patent 3,164,03 PROPELLANT GRAIN Allen L. Iilolzman, Palo Alto, and David Altman, Menlo Park, Calif assignors to United Aircraft Corporation, East Hartford, (Zones, a corporation of Delaware Filed May 6, 1963, Ser. No. 27%,166 4 Claims. (til. 1il2-98) This invention relates to a unique grain design suitable for rocket motors and, more particularly, to a hybrid grain design for substantially improving the regression rate.

One of the principal difficulties encountered in the development of hybrid rocket systems is the comparatively low burning rate of the solid phase. This is brought about by a boundary layer of combustion products which builds up between the liquid phase propellant and the solid phase forming an effective barrier to direct contact between the phases. For example, in a hybrid system wherein the solid phase is a fuel and the liquid phase is an oxidizer, the liquid oxidizer is injected into an axial perforation in the fuel grain whereupon the propellants react producing a high temperature effluent suitable for imparting thrust to the system. However, it has been found that the liquid phase propellant tends to core so that the concentration of oxidizer along the axis of the perforation is substantially higher than that found in the region in proximity to the fuel grain whereby combustion of the solid phase is decreased. To mitigate this effect considerable effort has been made to break up or diminish the fuel-rich boundary layer between the oxidizer-rich core and the grain surface. In the past, it has been suggested, for example, that certain additives be used in the solid phase to break up the boundary layer.

The present invention, however, contemplates a unique grain design wherein the coring effect of the oxidizer is virtually eliminated. To achieve this effect, the present invention contemplates employing a twisted planar body running the full length of the solid phase perforation whereby the liquid injectant and gases are forced to sweep in an efficient rotary motion across the surface of the solid phase in a manner so that the boundary layer is effective 1y diminished.

It is an objective, therefore, of the present invention to provide a unique grain design that will result in relatively high regression rates in hybrid engines.

Another object is to provide a unique grain design for use in engines where there are restrictions on the envelope on size allowed for the propulsion unit.

It is a further objective of the present invention to provide a unique grain design which will allow a high rate of erosive burning because of the forced impingement of combustion fluids against the propellant grain surface whereby the burning rate is substantially increased,

Other objects and advantages of this invention will become apparent from the following discussion, appended claims, and accompanying drawings in which:

FIG. 1 shows a hybrid rocket system employing a conventional cylindrical solid phase grain.

FIG. 2 is a longitudinal cross sectional view of the solid phase employing a unique grain design of the present invention.

FIG. 3 is a cross sectional view of FIG. 2 taken along section lines IIII.

Referring now to the drawings in which like reference Ice.

numerals have been used to designate like parts and initially to FIG. 1, there is illustrated a rocket motor comprised of cargo portion 10, liquid propellant storage area 11, the solid hybrid grain portion 12, and nozzle 13. In order to fire the rocket motor system,'valve 14 is opened to allow the liquid phase propellant 15 contained in tank 16 to pass through line 17 into injector 18 whereby a stream of the liquid propellant forms a spray 19 which enters into the perforation of grain 20. The liquid propellant 15 is then caused to ignite with the solid phase propellant producing a high temperature effluent 21 which exhausts through nozzle 13 giving thrust to the motor.

In a typical system Where the liquid propellant is an oxidizer and the solid phase is a fuel, it has been found that a high concentration of oxidizer will be located in the core area 22 which will pass out the nozzle without effectively combining with the solid phase fuel. It is believed that the combustion products evolving from the surface of grain form a boundary layer in region 23 which tends to act as a barrier to further access of oxidizer to the solid phase surface. The net effect is a substantial reduction in the regression rate as compared with solid propellant systems.

In order to decrease the coring effect and to substantially wipe away the boundary layer, the present invention calls for employing a twisted planar body 24 extending through the longitudinal axis of the cylindrical grain 20, as shown in FIG. 2. Consequently, as the gases flow through the helical convolutions of the twisted planar body 24, they are forced into the surface of the cylindrical grain 20 causing the boundary layer to be swept away. In the preferred embodiment shown therein, the twisted planar body or ribbon 24 is attached or bonded to the inside of the cylindrical grain 20 so that the total burning surface includes the two sides of the ribbon and the inner surface of the cylinder not in contact with the ribbon. The rate of twist of the ribbon and the ratio of ribbon thickness to the tube inside diameter will determine the relative gas velocities on the surface of the tube and ribbon. Since a planar body upon twisting will develop a longer path along its edges than it will in the center, the velocity of the gases will be a minimum at the center of the ribbon and a maximum at the cylinder or tube 20 and the radial extremities of the ribbon. Since the regression rate of the solid phase is proportional to the velocity of the fluid across the propellant surfaces and since the effect of the boundary layer is reduced, the regression rate is substantially increased.

In FIG. 3, a cross-sectional view is shown of the grain design in FIG. 2 illustrating that the ribbon portion 24 is substantially rectangular in cross section. However, the ribbon portion 24 may be made either concave or convex on either or both surfaces to permit substantially equal burn-through time at all points. Portion 24 may also contain surface grooves to channel the gases onto the grain surface at a more severe angle.

In a preferred embodiment of the present invention, the cylindrical body may be made from a suitable propellant material such as polymethyl methacrylate and the twisted ribbon 24 may be made of the same material. To facilitate production of a grain of this design, the cylinder portion 20 and the twisted ribbon portion 24 may be produced separately and then joined by adhesives or heat so that the materials at joint 25 become substantially continuous. However, the grain may be produced by casting entrance 1 m y y i #/sec. in.'- ilsed/in. in. 111. r /scc I in [see Grain A 0. G23 0. 60 0. 038 25 68. 0. 0132 0. 0045 Grain A 0. 023 0. (30 U. 038 ll) 27. 3 0. 0053 0. 0045 Grain B 0.023 0.60 0. 038 48. 2 O. 0247 0 0118 B/A' l l l 1 1. 77 4. 65 2. 62

to facilitate comparison, data for grain A is given which constitute the grain A performance adjusted for a length of 10 inches instead of the actual inches. From this table the marked increase in performance characteristics can be seen. For the same equivalent length, grain B, having 77 percent more grain surface area, exhibited an increase of 365 percent in the mass rate of ilow of the solid phase which can be attributed to the 162 percent improvement in the regression rate.

Although it is preferable to use a propellant for twisted member 24, it is possible to use non-propellant materials 4 such as stainless steel to cause the erosive burning and high regression rates produced by the wiping action of the high temperature gases across the propellant grain surface. Moreover, the unique grain design of the presl ent invention can be used in solid propellant motors where a very high degree of erosive burning is desired.

It is obvious that a variety of propellants may be used to produce suitable reaction motors employing the present grain design and the pitch and number of twists can be selectively adjusted in accordance with the burning characteristics of the propellants. In addition, twisted ribbons can be inserted in the perforations of a grain having more than one perforation, thereby increasing the regression rate.

Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that the foregoing discussion and accompanying drawings illustrate preferred embodiments of this invention and the latter is not to be unduly limited thereto.

Having thus described the invention, what is claimed is:

1. A propellant grain comprised of a substantially cylindrical body having an axial perforation and a twisted planar body extending longitudinally through and across said perforation whereby gases passing through the perforation traverse a substantially helical path.

2. A propellant grain as in claim 1 wherein said grain is the solid phase of a hybrid system.

3. A propellant grain as in claim 2 wherein said cylindrical body and said twisted planar body are contiguous.

4. A propellant grain as in claim 2 wherein said twisted planar body is integral with said cylindrical body.

References Cited by the Examiner UNITED STATES PATENTS 660,568 10/00 Gathrnann 102-98 2,661,692 12/53 Vegren 102-98 X 2,920,443 1/60 Higginson 10298 X 2,933,041 4/60 Ambrose l0298 3,068,641 12/62 FOX 35.6

OTHER REFERENCES Jet Propulsion, vol. 26, No. 2, pp. 102-105, Times for Interplanetary Trips, by Robert T. Jones.

SAMUEL FEINBERG, Primary Examiner.

BEIUAMIN A. BORCHELT, Examiner. 

1. A PROPELLANT GRAIN COMPRISED OF A SUBSTANTIALLY CYLINDRICAL BODY HAVING AN AXIAL PERFORATI AND A TWISTED PLANAR BODY EXTENDING LONGITUDINALLY THROUGH AND ACROSS SAID PERFORATION WHEREBY GASES PASSING THROUGH THE PERFORATION TRAVERSE A SUBSTANTIALLY JELICAL PATH. 